The present invention relates to new and improved single package, heat curable poly urea/urethane coating compositions. More particularly, it relates to aliphatic polyisocyanate-based poly urea/urethane coating compositions having improved storage stability, abrasion resistance, solvent resistance, weather resistance and good adhesion to substrates. The present invention also relates to articles coated with the new and improved coating compositions and to processes for preparing the compositions and coated substrates.
In this application, the term "aromatic isocyanate" refers to an organic isocyanate compound wherein the isocyanate group or groups is/are bonded directly to a carbon atom of an aromatic nucleus. By "aliphatic isocyanate" is meant an organic isocyanate compound wherein the isocyanate group or groups is/are bonded directly to an aliphatic carbon atom. "Polyaliphatic isocyanate" means a polyisocyanate with each isocyanate bonded directly to an aliphatic carbon atom.
Generally, polyurethane resins, upon curing, exhibit good flexibility, abrasion resistance, solvent resistance and good adhesion to a variety of substrates. Urethane resins have therefore been utilized in a variety of applications such as molding resins, potting compounds, adhesives, elastomers and coatings.
Early polyurethane coatings in the prior art are provided in the form of two component systems, wherein one component comprises an isocyanate compound and a second component comprises an hydroxyl-bearing compound. The polyurethane resins are formed by reaction of polyfunctional starting materials, the reaction between individual functional groups proceeding in accordance with the equation: EQU RNCO+R'OH.fwdarw.RNHCOOR'.
In use, the two components are mixed immediately prior to the coating application. The mixed product gels in a very short time period of, for example, from a few minutes to about 1 to 2 hours and thereafter cannot effectively be used and applied as a coating composition. The two component system coating compositions, having such a short coatability or use time before the onset of gelation, are very inconvenient because a separate mixing step is required immediately before application, and if a large area or a large number of articles need to be covered,several small batches of coating composition need to be prepared intermittently during the job to avoid wasting any unused composition which might gel before it can be applied to the substrate or substrates.
Moreover, the isocyanate groups are highly reactive with any compounds possessing reactive hydrogen which gives the coating system poor storage stability. The frequent result of high isocyanate reactivity is that the isocyanate groups pre-react with water, solvent and other materials, thereby decreasing the isocyanate functionality of the component even before it is admixed with the polyol component, which provides a less satisfactory coating. In addition, low molecular weight polyisocyanate compounds, and particularly toluene diisocyanate, are very volatile and may cause respiratory irritation at extremely low levels e.g., at concentrations on the order of 0.02 ppm in a closed environment, along with other attendant health hazards.
Early efforts at increasing the storage life of the two component compositions have included blocking the isocyanate groups by reacting them with phenols. Thereafter, when the two component system is ready for use, the polyol component is mixed with the phenol-blocked isocyanate and the mixture is coated on a substrate. The substrate and coating are then heated at elevated temperature, e.g., 350.degree.-400.degree. C., to dissociate the phenol from the isocyanate groups thereby liberating the isocyanate groups for reaction with the polyol to form urethanes. Phenol is evolved in the process, and this compound is extremely poisonous, so that extreme care in ventilation must be employed. Moreover, these coatings could only be used on substrates, such as metals and electrical wiring, which can withstand the high temperature treatments required to dissociate the phenols.
Early attempts were made to decrease the volatility and safety hazards of the low molecular weight isocyanate reactants. More particularly, two component compositions were improved by including the isocyanate component in the form of prepolymer adducts possessing polyisocyanate functionality. The prepolymer adducts effectively increase the molecular weight of the polyisocyanate, thereby reducing volatility and the health hazards associated therewith. Generally, the prepolymer adducts are prepared by reacting an excess of diisocyanate compound with a trihydric alcohol, e.g., trimethylolpropane, to form branched, higher molecular weight compounds possessing polyisocyanate functionality. These isocyanate prepolymer adducts may thereafter be reacted with the same or different polyols or reactive hydrogen containing compounds to form polyurethanes, polyureas, or polyesteramides via the following reactions: ##STR2## Generally, aromatic isocyanate compounds are used, such as tolylene diisocyanate, tolidine diisocyanate, and diphenylmethane diisocyanate. The isocyanate groups are still as reactive in the polymer adducts as in the monomeric precursors and therefore storage stability of these two component coating compositions is poor. After admixture with the polyol component, too rapid gelation times are still encountered. Further disadvantages of the two component polyisocyanate adduct coating compositions include the need for mixing the components prior to application and the poor weather resistance of aromatic isocyanate components e.g., they have a undesirable tendency for yellowing upon exposure to sunlight or other sources of ultraviolet light.
One component polyurethane coatings are also known. These systems are based mainly on stable isocyanate prepolymersobtained from tolylene diisocyanate and a hydroxy functional polyether. These coating compositions are moisture curable. They dry or cure by having the free isocyanate groups react with water by a reaction which proceeds through the unstable carbamic acid with carbon dioxide being eliminated to yield primary amine groups which further react with isocyanate groups to form ureas. The prepolymer for use in the one component package may be derived from other aromatic diisocyanates such as diphenylmethane diisocyanate, triphenylmethane triisocyanate and polyesters with hydroxyl terminal groups may be substituted for the polyether coreactant. These one component urethane coating compositions suffer from the same yellowing tendency upon weathering, as is common with all aromatic isocyanates.
In U.S. Pat. No. 3,296,156, one package polyurethane elastomer compositions having good storage stability and sufficiently long "pot" or use lives are described. These compositions may be cast or molded to form relatively large shaped articles. Such an elastomer composition is a mixture comprising an isocyanate terminated prepolymer adduct derived from an excess of an organic diisocyanate and a reactive hydrogen containing polymeric material having a molecular weight in excess of 500 selected from polyether glycols, polyester glycols and polyesteramides, wherein the molar ratio of diisocyanate groups to reactive hydrogen groups is from about 1.1 to 1 to about 4.0 to 1, and preferably at least about 2 to 1, and a crosslinker comprising 2-amino-2-methyl propanol.
It has now been discovered that new and improved single package heat curable polyurea/urethane coating compositions are provided in the form of a composition comprising a mixture or prepolymer of certain secondary or tertiary aliphatic isocyanates with a reactive hydrogen containing compound wherein the molar ratio of isocyanate groups to reactive hydrogens is from about 1:0.8 to about 1:1.2, respectively, and a minor effective amount of a polymerization catalyst.